- Email: [email protected]
Powdered activated carbon in an activated sludge treatment plant
Water Res. Vo[. IS. No. 2. pp. i33-13". 1984 Printed m Great Britain. All rights rese~'ed
0043-1354 8-1. $3.00-,-0.00 Copyright (~ 1q84 Pergamon Press Ltd
POWDERED ACTIVATED CARBON IN AN ACTIVATED SLUDGE T R E A T M E N T PLANT V. SPECCHIA and A. GIANETTO Dipartimento di Scienza dei Materiali ed Ingegneria Chimica Politecnico. Torino, Italia (Received .',larch 1982) Abstract-Addition of powdered activated carbon (PAC) to the aeration basin of an activated sludge treatment plant fed with dye-works waste waters increases the purifying capacity of the plant : removal efficiency rises from 55.8 to 75.6~o (COD) and from 78 to 98.5°0/BOD~) and the nitrification-denitrification capacity of the system also increases. The sludge growth parameters and the kinetic constant of biological oxidation were determined on the plant with and without PAC. The addition of PAC decreased overall sludge growth rate and the autooxidation factor, but increased the biological removal rate of the substrate by about one order of magnitude. Key words--activated sludge, powdered activated carbon (PAC), PAC bio-regeneration, aeration with PAC, dye-works wastewater, bio-oxidation enhancement, commercial scale, sludge growth parameters, bio-oxidation kinetics
NOMENCLATURE b F, F,. K, k k* Q S So t t' V X Y W /~
auto-oxidation factor, day - z volumetric loading factor, kg BOD~ m - s day- t sludge loading factor, kg BOD 5 kg- t MLSS day Michaelis-Menten constant, kg m-3 kinetic constant of substrate utilization, hpseudo-first-order kinetic constant, m 3 kg-t MLSS h-~ waste water flow rate, m s h- t substrate concentration in the effluent, kg m- s substrate concentration in waste water, kg m- 3 time, day time, h aeration basin volume, m s activated sludge concentration in the aeration basin, kg MLSS m -3 sludge growth factor, kg MLSS kg- ~ BODs daily sludge production per unit of basin volume, kg MLSS m -s day -t net specific sludge growth rate, day- t.
INTRODUCTION Direct addition of powdered activated carbon (PAC) to the aeration basin of an activated sludge plant has sometimes been employed in both the United States (Robertaccio et al., 1973; Thibault et al., 1977; Flynn and Stadnik, 1977) and Europe (Bettens, 1979). It is particularly indicated when poorly biodegradable organic substances are involved, since PAC favours their biological oxidation, and hence improves C O D and BOD5 removal efficiency. This appears to be due to bioregeneration of the PAC. Selected bacteria accustomed to the type of waste water develop within the pores of PAC and also degrade the more markedly bioresistant adsorbed substances ; the breakdown products desorb and new molecules can be adsorbed on vacant active sites (Bettens, 1979). Biological regeneration is thus a steady state bio-process.
F r o m the engineering standpoint, PAC addition is relatively simple and inexpensive. All that is needed is a system for metering the PAC aqueous suspension. The PAC concentration in the aeration basin is gradually brought to 0.7-1.5 kg m -3 and kept at this level by replacing the amount removed each day with the excess sludge. PRELIMINARY LABORATORY TESTS Laboratory tests were first carried out to study the effect of PAC addition on the activated sludge process in the presence of partly bioresistant substances. Waste water from a cotton and synthetic cloth dye-works was used. It contained dispersed, direct, acid and basic dyes ; anionic and nonionic detergents ; disperdent and anti-foam agents ; acetates, formiates and phosphates ; bleaching, wetting and retardant agents. Constant temperature respirometric tests were carried out with a Warburg type respirometer (total volume of 500 ml). Equal volumes of the same waste water sample were seeded with: 0.1 g 1-~ of activated sludge accustomed to the waste water (sample A); the same sludge concentration plus 0.2 or l g l-~ of Norit SA-5 PAC (sample B). PAC and waste water were also incubated alone. No appreciable effect has been noted, except unsteady adsorption of organic matter, depending on its initial concentration and the amount of PAC employed. Adsorption occurred in a short time after PAC addition. The oxygen uptake for samples A and B with waste waters taken up in two different days is shown in Figs. 1 and 2 and the ratio between oxygen uptake for sample B and sample A is plotted vs time in Fig. 3. After 20-40 h induction, presumably dependent on differences in the time required to trigger biological 133
134
V. SPECCHIAand A. GtANETTO 6 -4 •
a
7
4::
E
7
~, •
7 •
7
7
•
•
?
=
8 • l
•
Waste
water: + O l q l" M L S S
9
Waste
water + 0 I 0 L ' I M L S S * 0 2
•
Waste
water + 0 I g I," I M L S S • ~ 0 l " l P A C
[
2 O
OL"
pAC
I
50
i
iO0
150
(h)
Time
Fig. 1. Respirometric tests : oxygen uptake with and without PAC. activity, the ratio settled down to a value that increased in function of the PAC concentration. The greater O : uptake of samples B was about 20 and 75% for PAC addition of 0.2 and 1 gl -~ respectively. It may be supposed that increasing PAC concentrations permit larger populations of bacteria in PAC macropores. Since the relationship between PAC concentration and the rate of biodegradation does not seem to be linear, the lack of substrata probably imposes a limitation on bacterial growth. This fact must be kept in mind when working on an industrial scale to make sure that satisfactory increases in removal efficiency are coupled with reasonable running costs when PAC is added.
runs were carried out. The first unsteady-state run ',,,as performed without PAC. After about 2 months, PAC was gradually introduced into the aeration basin over the course of about 1 week to reach a PAC concentration of about 0.8 kg m -~. Inlet and outlet BOD~ and MLSS Ibiological sludge only, i.e. without PAC) concentrations were periodically determined. Owing to the slowness of the biological phenomena, outlet BOD~ and MLSS concentrations varied very slowly over periods of at least 5-6 days ; constant values, equal to the mean experimental value for each 5-6 day period, were therefore assumed. Next the following steady-state conditions were maintained for about 2 months without PAC (PAC was removed from the plant by not replacing that discharged with the sludge wastage) : mean organic loading : approx. 50 kg BOD~ day - t MLSS concentration: 2.5-3.4 kg m - 3 sludge recycle ratio : approx. 1 mean excess sludge rate: 18 kg day - ~, daily. Subsequently PAC was again added to the aeration basin. The same steady-state conditions were maintained, except that the mean excess sludge rate was lower (15 kg d a y - ~). A suitable quantity of PAC was added once a day after the excess sludge had been wasted from the plant to ensure a constant concentration of about 0.8 kg m-3. During these last two runs, significant waste water, effluent and aeration basin parameters were determined periodically. CHARACTERISTIC
TESTS
PERFORMED
ON
AN
INDUSTRIAL
OXIDATION
SCALE
The laboratory results showed that PAC could be usefully employed for dye-works waste water. It was then used in an industrial treatment plant fed with this type of influent. The block diagram of the plant is shown in Fig. 4. The waste water flow rate was about 100 m s d a y - t and the volume of the equalization and aeration basin were 150 and 200 m s respectively. The plant worked for about 4 months at as constant an organic load as possible without sludge wastage, i.e. allowing the biomass concentration to increase. Two
BIOLOGICAL PARAMETERS
Sludge 9rowth rate The results obtained during the unsteady state runs were used to determine the biomass growth parameters, either with or without PAC. Since the amount of sludge lost through the ctarifier overflow weir is negligible, the net growth of sludge in the basin, when no sludge is discharged, can be written in the form : dx
= _ Y dS
dr'
x
t I :,
dr'
I0 8
'E
v 0
•
4
v v ~
•
•
v
v
0
O
0
D
o
D
0 Q
~j
6
2
0 0
0
0 0
0 0
O
0
o.
iK ~~
0
Q
~
{3 Waste
nd ~
o o
8 0
6 0
am
o
4 e
0 2 gt"s p A C
o •
l g c -~ P A C
F tOO
,,
o -Q ~o
2
wafer ~, 0 I g t " t MLSS
O Waste
w a l e r e OI
g t - I MLSS + 0.2 g i ' l P A C
E
t
w~er ÷0.1
Q L" 1 M L S S
~
Waste
8
6
I
I
50
JO0 Time
* I gt-' ,
PAC
nO°
1 50
,
I !50
J
;50
( h )
Fig. 2. Respirometric tests : oxygen uptake with and without PAC.
Time
th)
Fig. 3. Oxygen consumption (respirometric runs) vs time, with and without PAC (different symbols for the same PAC concentration concern runs at different timest
PAC in an activated sludge treatment plant
135
PAC increases the bio-oxidation of the organic substance, but reduces the overall growth of sludge, even though the auto-oxidation factor is diminished.
W&STE WATER i t
Rate of biological removal of the substrate
I FeedmgOr
This can be expressed in the form (Lawrence and McCarty, 1970):
constont /
rote
f
dS kSX dt = K~ + S "
INeutrolizotionI
~
SludGe reclrcuiotion
J-
PAC odditionJ
!
1
]Sludge thickening j t
I ChJorination,J ,
i
Sludge dewatering I by centrifugot~on
EFFLUENT
EXCESS SLUDGE
Fig, 4. Block diagram of the treatment plant.
Division of both sides of the equation by X gives :
/~
AX 1 = At---s'~ =
--
AS 1 Y~tTt,'~ - - b.
(2')
(3)
It was found that the COD, BOD s and total suspended solid values for samples taken at several points in the basin were practically the same (mean d e v i a t i o n + 2 ~ from the mean in all cases} and the mean basin COD and BODs values were virtually the same as those observed in the overflow weir emuent. In addition, the mean substrate concentration in the basin (expressed as BODs) was always very low. Two assumptions were therefore drawn : (a) the basin behaves like a perfectly mixed reactor, and all substrate utilization occurs in the basin; (b) the substrate concentration in the basin, S, is negligible with respect to Ks. In addition, for the sake of simplicity, kinetic limitations due to substrate mass transfer into the PAC macropores were assumed to be negligible.
For not too large X variations, equation (2) can be written in terms of finite differences : AX 1 /~ = At' X
_yAS.l_b" A(
(2')
0.18 -
X
If the specific sludge growth rate, ,u, is plotted as a function of the sludge loading factor, F~ = --(AS/At').(1/X), one obtains a straight line with a slope Y and intercept - b . These parameters are characteristic of the type of substrate present in the waste water, and have been reported for various types of waste water (Eckenfelder, 1966; Lawrence and McCarty, 1970). In Fig. 5, y is plotted as a function of F c, with and without PAC. The points are calculated from the basin sludge concentration and inlet and outlet BOD5 values, the waste water flow rate and basin volume being known. Table 1 compares Y and b values obtained from our data points by linear regression with those reported for urban sewage (Eckenfelder, 1966; Lawrence and McCarty, 1970). Without PAC, the differences are minimal, whereas a reduction takes place, mainly in b, after PAC introduction. In other words, the presence of PAC reduces the growth of activated sludge, as can be seen in Fig. 6, where the daily production of sludge per unit of basin volume, W (calculated as the difference between the final and initial MLSS concentration for each 5--6 day period, divided by the days of observation), is plotted against the volumetric loading factor F~, Figures 5 and 6 seem to demonstrate that
0.15
•
Without
•
With
PAC
PAC
0.12
J'
0.09
°°6r ; 0.03 I
L
I ~
0
-0.03
- 0,06
I
0
o.i
I
I
0.2
kg SS, day
03
J
Fig. 5. Net specific sludge growth rate vs sludge loading factor, with and without PAC.
V. SPECCHIA and A. GIANETTO
136
Table I Without P A C
With PAC
Urban sewage+
0.67 0,043 0.046 "+"30%
0.48 0.018 0.32-_ 16"~,
0.50-0.67 0,048-0.055 I0.6
Y (kg MLSS kg- t BOD~I b tkg MLSS k g - t M LSS day- t l k" (mS kg -t MLSSh -~)
?Eckenfelder (1966); Lawrence and McCarty (1970); Benedek and Horvath (1967).
Assumption (b) allows us to simplify equation (3) to the form : (4}
- dS/dt = k*SX
where k* = k/K3 is equivalent to a pseudo-first-order kinetic constant. Assumption (a) can be used to write the substrate mass balance for the basin in the form : (5]
Q ( S o - S) = V k * S X
which enables k* to be determined. Mean k* values (on BODs basis) in the plant with and without PAC are shown in Table I, together with that for urban sewage (Benedek and Horvath, 1967); the inlet and outlet BOD~ and MLSS concentration measured during the two steady-state runs have been employed. It can be seen that approximately one order of magnitude separates the three k* values. Even though the addition of PAC results in effluent substrate concentrations comparable with those found in the effluent from an urban sewage treatment plant, the rate of biological removal is less. Therefore, contrary to the assumption made, the bacterial activity in the PAC
0.15
•
Without
•
With
PAC PAC /
I
i
I
I
~
/e
o.,
•
../,
pores may be partly controlled by substrate mass transfer into the pores.
/
o/
/ /O
BIOLOGICAL REMOVAL EFFICIENCY
The mean values ofsome significant waste water and effluent parameters, determined during the steadystate working period with and without PAC. are shown in Table 2. In Figs. 7 and 8, the COD and BOD~ pattern of the waste water and effluent are plotted vs time before and after PAC addition (indicated by the dashed line). PAC introduction led to a marked improvement in removal capacity, particularly with respect to organic substances. After PAC, the mean COD and BOD5 removal efficiency rose from 55.8 to 75.C/o and from 78 to 98.5~o respectively. Moreover the amount of COD removed biologically increased on an average from 1.44 to 1.95 times the initial BOD 5. The extra COD removed in the presence of PAC has been probably biodegradate. During steady operation, in fact, no appreciable effect of physical adsorption on PAC can be expected, while higher oxygen uptake is induced by PAC. It was not possible to quantify the oxygen uptake in the aeration basin ; the only evidence was a remarkable 0 2 concentration gradient in the aeration basin when using PAC: at the bottom it was about 1 mg 1- ~, as opposed to 2-3 mg l- ~ below the liquid surface. Two conclusions can be drawn : the addition of PAC leads to the removal of substances that were hard to biodegrade; BOD~ values of the influent obtained in the laboratory do not reflect the true removal capacity
•/.
0/ /
.,J 0.05
/
/
Table 2
¢•
/% Waste water O O.I
I 0.2
1 03
I Oa
I" k9 BODsrweov=d" . L m 3 day
Fig. 6. D a i l y sludge net g r o w t h rate vs v o l u m e t r i c l o a d i n g factor, with and w i t h o u t PAC,
pH COD (rag I- '! BODs (rag I- ') NH 3 nitrogen (rag 1- 1 N) NO~ nitrogen (rag I - ' N) NO3 nitrogen (rag l- t N)
Phosphates{rag MBAS [rag I-~)
I - ~)
5 1293 500 25 -13 26
90
Emuent before PAC addition 7-7.5 572 I10 13
Effluent alter PAC addition 7-7.5 3 ]6
26
abs. abs. 3.2
14
23
73
t.7
PAC in an activated sludge treatment plant I
L
' :" 103 III I
•
""
",A'"
"
•ll
•
1I
It °oo #oO°
O 0 ~ 3
i0 • COO waste
0
water
0
0
0
°
°O
0 ¢ 0 0 effluent I ,
n []
[ [
t
0
4O
I
I
I
80
120
160
Time
( day )
Fig. 7. Waste water and effluent COD vs time before and after PAC addition (vertical dotted line).
!i
I
o
?o%#o o 0
I
0 0
0
0
CONCLUSIONS
I
I I ,1~
I
T
O
6 3 water
0
BOD 5
•
BOD 5 e f f l u e n t
waste
io J _
•
¢.
6
••¢
3 0
The values in the last column of Table 2 lie within the limits permitted for sewers in Table C of the relevant Italian legislation(Law No. 650, 1979). With the exception of the COD value, they are also below the maximum limits for discharge into surface waters (Table A of the same Law). In a search for ways of further reducing the COD value to meet future legislative requirements, physical adsorption tests were run at laboratory scale on biopurified effluent from the plant. Several types of granular activated carbon were used. The results were unsatisfactory, since adsorption capacities lower than 20 mg g - t were obtained in all cases. Spectrophotometric analysis of effluent samples revealed the presence of large amounts of highly polar organic substances (constituting more than 50°,0 of the COD value when expressed as phenol equivalent). Activated carbon is a very poor adsorber of polar substances due to their marked affinity for water (Hassler, 1963).
I
oo
OO
E
137
I
I 50
I00 T~me
1
150
( day )
Fig. 8. Waste water and effluent BODs vs time before and after PAC addition (vertical dotted line).
of the plant. Even though BOD 5 measurements were always taken on samples seeded with sludge produced in the plant (so that when PAC was added the seed also contained activated carbon) the actual purifying capacity of the aeration basin was much greater than expected from the measured BOD s values of the feed. Table 2 also shows that PAC results in almost complete nitrification of ammoniacal N and nitrites into nitrates and that the latter were greatly denitrifled ; this larger nitrogen disappearance began just 1 week after PAC addition. The lower 02 concentration at the bottom of the aeration basin probably explains this phenomenon. On the other hand, the disappearance of N may be due to the high concentration of nitrifying-denitrifying bacteria on the PAC surface (Bettens, 1979).
Respirometric runs at laboratory scale and direct employment in an activated sludge treatment plant handling waste water from a dye-works, showed that the addition of PAC to the sludge increases its purifying capacity in a considerable amount, both in terms of the oxidation of organic substances (removal efficiency of COD and BOD 5 increased from 55,8 to 75.6% and from 78 to 98.5% respectively) and of ammoniacal and nitric nitrogen removal. The characteristic parameters of sludge growth and the kinetic constant of biological oxidation were also determined on the plant, with and without PAC. The addition of PAC reduced sludge growth, but increased the biological removal rate by about one order of magnitude, though the value thus obtained was still one order of magnitude less than that reported for an urban sewage treatment plant. Lastly, the autooxidation factor was also noticeably reduced. Acknowledgements--We wish to thank the Chiavazza Company for allowing us to use the data from treatment plant and Mr E. Marras of Norit Italia for supplying us with PAC and granular activated carbon.
REFERENCES
Benedek P. and Horvath I. (1967) Water Res. l, 663. Bettens L. (1979) Effl. War. Treat. J. 129. Eckenfelder W. W. Jr (1976) Industrial Water Pollution Control. McGraw-Hill, New York. Flynn B. P, and Stadnik J. G. (1977) 50th Annual Water Pollution Control Federation Conference, Philadelphia, PA. Hassler J. W. (1963) Actirated Carbon. Chemical Publishing Co., New York. Lawrence A. W. and McCarty P. L. (1970)J. sanit. Engng. Div. 757. Robertaccio F. L., Hutton D. G., Grulich G. and Glotzer H. L, (1973) AIChE National Meeting, Dallas, TX. Thibault G. T., Tracy K. D. and Wilkinson J. B. (1977) Hydrocarbon Process. 143.
0043-1354 8-1. $3.00-,-0.00 Copyright (~ 1q84 Pergamon Press Ltd
POWDERED ACTIVATED CARBON IN AN ACTIVATED SLUDGE T R E A T M E N T PLANT V. SPECCHIA and A. GIANETTO Dipartimento di Scienza dei Materiali ed Ingegneria Chimica Politecnico. Torino, Italia (Received .',larch 1982) Abstract-Addition of powdered activated carbon (PAC) to the aeration basin of an activated sludge treatment plant fed with dye-works waste waters increases the purifying capacity of the plant : removal efficiency rises from 55.8 to 75.6~o (COD) and from 78 to 98.5°0/BOD~) and the nitrification-denitrification capacity of the system also increases. The sludge growth parameters and the kinetic constant of biological oxidation were determined on the plant with and without PAC. The addition of PAC decreased overall sludge growth rate and the autooxidation factor, but increased the biological removal rate of the substrate by about one order of magnitude. Key words--activated sludge, powdered activated carbon (PAC), PAC bio-regeneration, aeration with PAC, dye-works wastewater, bio-oxidation enhancement, commercial scale, sludge growth parameters, bio-oxidation kinetics
NOMENCLATURE b F, F,. K, k k* Q S So t t' V X Y W /~
auto-oxidation factor, day - z volumetric loading factor, kg BOD~ m - s day- t sludge loading factor, kg BOD 5 kg- t MLSS day Michaelis-Menten constant, kg m-3 kinetic constant of substrate utilization, hpseudo-first-order kinetic constant, m 3 kg-t MLSS h-~ waste water flow rate, m s h- t substrate concentration in the effluent, kg m- s substrate concentration in waste water, kg m- 3 time, day time, h aeration basin volume, m s activated sludge concentration in the aeration basin, kg MLSS m -3 sludge growth factor, kg MLSS kg- ~ BODs daily sludge production per unit of basin volume, kg MLSS m -s day -t net specific sludge growth rate, day- t.
INTRODUCTION Direct addition of powdered activated carbon (PAC) to the aeration basin of an activated sludge plant has sometimes been employed in both the United States (Robertaccio et al., 1973; Thibault et al., 1977; Flynn and Stadnik, 1977) and Europe (Bettens, 1979). It is particularly indicated when poorly biodegradable organic substances are involved, since PAC favours their biological oxidation, and hence improves C O D and BOD5 removal efficiency. This appears to be due to bioregeneration of the PAC. Selected bacteria accustomed to the type of waste water develop within the pores of PAC and also degrade the more markedly bioresistant adsorbed substances ; the breakdown products desorb and new molecules can be adsorbed on vacant active sites (Bettens, 1979). Biological regeneration is thus a steady state bio-process.
F r o m the engineering standpoint, PAC addition is relatively simple and inexpensive. All that is needed is a system for metering the PAC aqueous suspension. The PAC concentration in the aeration basin is gradually brought to 0.7-1.5 kg m -3 and kept at this level by replacing the amount removed each day with the excess sludge. PRELIMINARY LABORATORY TESTS Laboratory tests were first carried out to study the effect of PAC addition on the activated sludge process in the presence of partly bioresistant substances. Waste water from a cotton and synthetic cloth dye-works was used. It contained dispersed, direct, acid and basic dyes ; anionic and nonionic detergents ; disperdent and anti-foam agents ; acetates, formiates and phosphates ; bleaching, wetting and retardant agents. Constant temperature respirometric tests were carried out with a Warburg type respirometer (total volume of 500 ml). Equal volumes of the same waste water sample were seeded with: 0.1 g 1-~ of activated sludge accustomed to the waste water (sample A); the same sludge concentration plus 0.2 or l g l-~ of Norit SA-5 PAC (sample B). PAC and waste water were also incubated alone. No appreciable effect has been noted, except unsteady adsorption of organic matter, depending on its initial concentration and the amount of PAC employed. Adsorption occurred in a short time after PAC addition. The oxygen uptake for samples A and B with waste waters taken up in two different days is shown in Figs. 1 and 2 and the ratio between oxygen uptake for sample B and sample A is plotted vs time in Fig. 3. After 20-40 h induction, presumably dependent on differences in the time required to trigger biological 133
134
V. SPECCHIAand A. GtANETTO 6 -4 •
a
7
4::
E
7
~, •
7 •
7
7
•
•
?
=
8 • l
•
Waste
water: + O l q l" M L S S
9
Waste
water + 0 I 0 L ' I M L S S * 0 2
•
Waste
water + 0 I g I," I M L S S • ~ 0 l " l P A C
[
2 O
OL"
pAC
I
50
i
iO0
150
(h)
Time
Fig. 1. Respirometric tests : oxygen uptake with and without PAC. activity, the ratio settled down to a value that increased in function of the PAC concentration. The greater O : uptake of samples B was about 20 and 75% for PAC addition of 0.2 and 1 gl -~ respectively. It may be supposed that increasing PAC concentrations permit larger populations of bacteria in PAC macropores. Since the relationship between PAC concentration and the rate of biodegradation does not seem to be linear, the lack of substrata probably imposes a limitation on bacterial growth. This fact must be kept in mind when working on an industrial scale to make sure that satisfactory increases in removal efficiency are coupled with reasonable running costs when PAC is added.
runs were carried out. The first unsteady-state run ',,,as performed without PAC. After about 2 months, PAC was gradually introduced into the aeration basin over the course of about 1 week to reach a PAC concentration of about 0.8 kg m -~. Inlet and outlet BOD~ and MLSS Ibiological sludge only, i.e. without PAC) concentrations were periodically determined. Owing to the slowness of the biological phenomena, outlet BOD~ and MLSS concentrations varied very slowly over periods of at least 5-6 days ; constant values, equal to the mean experimental value for each 5-6 day period, were therefore assumed. Next the following steady-state conditions were maintained for about 2 months without PAC (PAC was removed from the plant by not replacing that discharged with the sludge wastage) : mean organic loading : approx. 50 kg BOD~ day - t MLSS concentration: 2.5-3.4 kg m - 3 sludge recycle ratio : approx. 1 mean excess sludge rate: 18 kg day - ~, daily. Subsequently PAC was again added to the aeration basin. The same steady-state conditions were maintained, except that the mean excess sludge rate was lower (15 kg d a y - ~). A suitable quantity of PAC was added once a day after the excess sludge had been wasted from the plant to ensure a constant concentration of about 0.8 kg m-3. During these last two runs, significant waste water, effluent and aeration basin parameters were determined periodically. CHARACTERISTIC
TESTS
PERFORMED
ON
AN
INDUSTRIAL
OXIDATION
SCALE
The laboratory results showed that PAC could be usefully employed for dye-works waste water. It was then used in an industrial treatment plant fed with this type of influent. The block diagram of the plant is shown in Fig. 4. The waste water flow rate was about 100 m s d a y - t and the volume of the equalization and aeration basin were 150 and 200 m s respectively. The plant worked for about 4 months at as constant an organic load as possible without sludge wastage, i.e. allowing the biomass concentration to increase. Two
BIOLOGICAL PARAMETERS
Sludge 9rowth rate The results obtained during the unsteady state runs were used to determine the biomass growth parameters, either with or without PAC. Since the amount of sludge lost through the ctarifier overflow weir is negligible, the net growth of sludge in the basin, when no sludge is discharged, can be written in the form : dx
= _ Y dS
dr'
x
t I :,
dr'
I0 8
'E
v 0
•
4
v v ~
•
•
v
v
0
O
0
D
o
D
0 Q
~j
6
2
0 0
0
0 0
0 0
O
0
o.
iK ~~
0
Q
~
{3 Waste
nd ~
o o
8 0
6 0
am
o
4 e
0 2 gt"s p A C
o •
l g c -~ P A C
F tOO
,,
o -Q ~o
2
wafer ~, 0 I g t " t MLSS
O Waste
w a l e r e OI
g t - I MLSS + 0.2 g i ' l P A C
E
t
w~er ÷0.1
Q L" 1 M L S S
~
Waste
8
6
I
I
50
JO0 Time
* I gt-' ,
PAC
nO°
1 50
,
I !50
J
;50
( h )
Fig. 2. Respirometric tests : oxygen uptake with and without PAC.
Time
th)
Fig. 3. Oxygen consumption (respirometric runs) vs time, with and without PAC (different symbols for the same PAC concentration concern runs at different timest
PAC in an activated sludge treatment plant
135
PAC increases the bio-oxidation of the organic substance, but reduces the overall growth of sludge, even though the auto-oxidation factor is diminished.
W&STE WATER i t
Rate of biological removal of the substrate
I FeedmgOr
This can be expressed in the form (Lawrence and McCarty, 1970):
constont /
rote
f
dS kSX dt = K~ + S "
INeutrolizotionI
~
SludGe reclrcuiotion
J-
PAC odditionJ
!
1
]Sludge thickening j t
I ChJorination,J ,
i
Sludge dewatering I by centrifugot~on
EFFLUENT
EXCESS SLUDGE
Fig, 4. Block diagram of the treatment plant.
Division of both sides of the equation by X gives :
/~
AX 1 = At---s'~ =
--
AS 1 Y~tTt,'~ - - b.
(2')
(3)
It was found that the COD, BOD s and total suspended solid values for samples taken at several points in the basin were practically the same (mean d e v i a t i o n + 2 ~ from the mean in all cases} and the mean basin COD and BODs values were virtually the same as those observed in the overflow weir emuent. In addition, the mean substrate concentration in the basin (expressed as BODs) was always very low. Two assumptions were therefore drawn : (a) the basin behaves like a perfectly mixed reactor, and all substrate utilization occurs in the basin; (b) the substrate concentration in the basin, S, is negligible with respect to Ks. In addition, for the sake of simplicity, kinetic limitations due to substrate mass transfer into the PAC macropores were assumed to be negligible.
For not too large X variations, equation (2) can be written in terms of finite differences : AX 1 /~ = At' X
_yAS.l_b" A(
(2')
0.18 -
X
If the specific sludge growth rate, ,u, is plotted as a function of the sludge loading factor, F~ = --(AS/At').(1/X), one obtains a straight line with a slope Y and intercept - b . These parameters are characteristic of the type of substrate present in the waste water, and have been reported for various types of waste water (Eckenfelder, 1966; Lawrence and McCarty, 1970). In Fig. 5, y is plotted as a function of F c, with and without PAC. The points are calculated from the basin sludge concentration and inlet and outlet BOD5 values, the waste water flow rate and basin volume being known. Table 1 compares Y and b values obtained from our data points by linear regression with those reported for urban sewage (Eckenfelder, 1966; Lawrence and McCarty, 1970). Without PAC, the differences are minimal, whereas a reduction takes place, mainly in b, after PAC introduction. In other words, the presence of PAC reduces the growth of activated sludge, as can be seen in Fig. 6, where the daily production of sludge per unit of basin volume, W (calculated as the difference between the final and initial MLSS concentration for each 5--6 day period, divided by the days of observation), is plotted against the volumetric loading factor F~, Figures 5 and 6 seem to demonstrate that
0.15
•
Without
•
With
PAC
PAC
0.12
J'
0.09
°°6r ; 0.03 I
L
I ~
0
-0.03
- 0,06
I
0
o.i
I
I
0.2
kg SS, day
03
J
Fig. 5. Net specific sludge growth rate vs sludge loading factor, with and without PAC.
V. SPECCHIA and A. GIANETTO
136
Table I Without P A C
With PAC
Urban sewage+
0.67 0,043 0.046 "+"30%
0.48 0.018 0.32-_ 16"~,
0.50-0.67 0,048-0.055 I0.6
Y (kg MLSS kg- t BOD~I b tkg MLSS k g - t M LSS day- t l k" (mS kg -t MLSSh -~)
?Eckenfelder (1966); Lawrence and McCarty (1970); Benedek and Horvath (1967).
Assumption (b) allows us to simplify equation (3) to the form : (4}
- dS/dt = k*SX
where k* = k/K3 is equivalent to a pseudo-first-order kinetic constant. Assumption (a) can be used to write the substrate mass balance for the basin in the form : (5]
Q ( S o - S) = V k * S X
which enables k* to be determined. Mean k* values (on BODs basis) in the plant with and without PAC are shown in Table I, together with that for urban sewage (Benedek and Horvath, 1967); the inlet and outlet BOD~ and MLSS concentration measured during the two steady-state runs have been employed. It can be seen that approximately one order of magnitude separates the three k* values. Even though the addition of PAC results in effluent substrate concentrations comparable with those found in the effluent from an urban sewage treatment plant, the rate of biological removal is less. Therefore, contrary to the assumption made, the bacterial activity in the PAC
0.15
•
Without
•
With
PAC PAC /
I
i
I
I
~
/e
o.,
•
../,
pores may be partly controlled by substrate mass transfer into the pores.
/
o/
/ /O
BIOLOGICAL REMOVAL EFFICIENCY
The mean values ofsome significant waste water and effluent parameters, determined during the steadystate working period with and without PAC. are shown in Table 2. In Figs. 7 and 8, the COD and BOD~ pattern of the waste water and effluent are plotted vs time before and after PAC addition (indicated by the dashed line). PAC introduction led to a marked improvement in removal capacity, particularly with respect to organic substances. After PAC, the mean COD and BOD5 removal efficiency rose from 55.8 to 75.C/o and from 78 to 98.5~o respectively. Moreover the amount of COD removed biologically increased on an average from 1.44 to 1.95 times the initial BOD 5. The extra COD removed in the presence of PAC has been probably biodegradate. During steady operation, in fact, no appreciable effect of physical adsorption on PAC can be expected, while higher oxygen uptake is induced by PAC. It was not possible to quantify the oxygen uptake in the aeration basin ; the only evidence was a remarkable 0 2 concentration gradient in the aeration basin when using PAC: at the bottom it was about 1 mg 1- ~, as opposed to 2-3 mg l- ~ below the liquid surface. Two conclusions can be drawn : the addition of PAC leads to the removal of substances that were hard to biodegrade; BOD~ values of the influent obtained in the laboratory do not reflect the true removal capacity
•/.
0/ /
.,J 0.05
/
/
Table 2
¢•
/% Waste water O O.I
I 0.2
1 03
I Oa
I" k9 BODsrweov=d" . L m 3 day
Fig. 6. D a i l y sludge net g r o w t h rate vs v o l u m e t r i c l o a d i n g factor, with and w i t h o u t PAC,
pH COD (rag I- '! BODs (rag I- ') NH 3 nitrogen (rag 1- 1 N) NO~ nitrogen (rag I - ' N) NO3 nitrogen (rag l- t N)
Phosphates{rag MBAS [rag I-~)
I - ~)
5 1293 500 25 -13 26
90
Emuent before PAC addition 7-7.5 572 I10 13
Effluent alter PAC addition 7-7.5 3 ]6
26
abs. abs. 3.2
14
23
73
t.7
PAC in an activated sludge treatment plant I
L
' :" 103 III I
•
""
",A'"
"
•ll
•
1I
It °oo #oO°
O 0 ~ 3
i0 • COO waste
0
water
0
0
0
°
°O
0 ¢ 0 0 effluent I ,
n []
[ [
t
0
4O
I
I
I
80
120
160
Time
( day )
Fig. 7. Waste water and effluent COD vs time before and after PAC addition (vertical dotted line).
!i
I
o
?o%#o o 0
I
0 0
0
0
CONCLUSIONS
I
I I ,1~
I
T
O
6 3 water
0
BOD 5
•
BOD 5 e f f l u e n t
waste
io J _
•
¢.
6
••¢
3 0
The values in the last column of Table 2 lie within the limits permitted for sewers in Table C of the relevant Italian legislation(Law No. 650, 1979). With the exception of the COD value, they are also below the maximum limits for discharge into surface waters (Table A of the same Law). In a search for ways of further reducing the COD value to meet future legislative requirements, physical adsorption tests were run at laboratory scale on biopurified effluent from the plant. Several types of granular activated carbon were used. The results were unsatisfactory, since adsorption capacities lower than 20 mg g - t were obtained in all cases. Spectrophotometric analysis of effluent samples revealed the presence of large amounts of highly polar organic substances (constituting more than 50°,0 of the COD value when expressed as phenol equivalent). Activated carbon is a very poor adsorber of polar substances due to their marked affinity for water (Hassler, 1963).
I
oo
OO
E
137
I
I 50
I00 T~me
1
150
( day )
Fig. 8. Waste water and effluent BODs vs time before and after PAC addition (vertical dotted line).
of the plant. Even though BOD 5 measurements were always taken on samples seeded with sludge produced in the plant (so that when PAC was added the seed also contained activated carbon) the actual purifying capacity of the aeration basin was much greater than expected from the measured BOD s values of the feed. Table 2 also shows that PAC results in almost complete nitrification of ammoniacal N and nitrites into nitrates and that the latter were greatly denitrifled ; this larger nitrogen disappearance began just 1 week after PAC addition. The lower 02 concentration at the bottom of the aeration basin probably explains this phenomenon. On the other hand, the disappearance of N may be due to the high concentration of nitrifying-denitrifying bacteria on the PAC surface (Bettens, 1979).
Respirometric runs at laboratory scale and direct employment in an activated sludge treatment plant handling waste water from a dye-works, showed that the addition of PAC to the sludge increases its purifying capacity in a considerable amount, both in terms of the oxidation of organic substances (removal efficiency of COD and BOD 5 increased from 55,8 to 75.6% and from 78 to 98.5% respectively) and of ammoniacal and nitric nitrogen removal. The characteristic parameters of sludge growth and the kinetic constant of biological oxidation were also determined on the plant, with and without PAC. The addition of PAC reduced sludge growth, but increased the biological removal rate by about one order of magnitude, though the value thus obtained was still one order of magnitude less than that reported for an urban sewage treatment plant. Lastly, the autooxidation factor was also noticeably reduced. Acknowledgements--We wish to thank the Chiavazza Company for allowing us to use the data from treatment plant and Mr E. Marras of Norit Italia for supplying us with PAC and granular activated carbon.
REFERENCES
Benedek P. and Horvath I. (1967) Water Res. l, 663. Bettens L. (1979) Effl. War. Treat. J. 129. Eckenfelder W. W. Jr (1976) Industrial Water Pollution Control. McGraw-Hill, New York. Flynn B. P, and Stadnik J. G. (1977) 50th Annual Water Pollution Control Federation Conference, Philadelphia, PA. Hassler J. W. (1963) Actirated Carbon. Chemical Publishing Co., New York. Lawrence A. W. and McCarty P. L. (1970)J. sanit. Engng. Div. 757. Robertaccio F. L., Hutton D. G., Grulich G. and Glotzer H. L, (1973) AIChE National Meeting, Dallas, TX. Thibault G. T., Tracy K. D. and Wilkinson J. B. (1977) Hydrocarbon Process. 143.